Cutaneous nerve stimulation by psoralen-ultraviolet A therapy: an ultrastructural study

Anatomical contacts between sensory neurons and epidermal cells: an unrecognized anatomical network for neuro-immuno-cutaneous crosstalk

Sensory neurons innervating the skin are conventionally thought to be the sole transducers of touch, temperature, pain and itch. However, recent studies have shown that keratinocytes - like Merkel cells - act as sensory transducers, whether for innocuous or noxious mechanical, thermal or chemical stimuli, and communicate with intraepidermal free nerve endings via chemical synaptic contacts. This paradigm shift leads to consideration of the whole epidermis as a sensory epithelium. Sensory neurons additionally function as an efferent system. Through the release of neuropeptides in intimate neuroepidermal contact areas, they contribute to epidermal homeostasis and to the pathogenesis of inflammatory skin diseases. To counteract the dogma regarding neurocutaneous interactions, seen exclusively from the perspective of soluble and spreading mediators, this review highlights the essential contribution of the unrecognized anatomical contacts between sensory neurons and epidermal cells (keratinocytes, melanocytes, Langerhans cells and Merkel cells), which take part in the reciprocal dialogue between the skin, nervous system and immune system.

On the development of neurocutaneous units--implications for the histogenesis of congenital, acquired, and dysplastic nevi

This study of spontaneous abortions and fetal deaths in utero used immunostains to evaluate the structure of developing cutaneous nerves. Melan-A immunostains were also used to screen 25 cases of grossly normal fetal skin for occult fetal nevi. Discrete portions of epidermis were generally supplied by branches emanating from regularly spaced deep cutaneous nerves, producing a wedge shape, interpreted as neurocutaneous units (NCU). Deeper nerves embraced broader portions of epidermis. Some nerves ran parallel to epidermis, especially near the superficial vascular plexus at the junction of superficial and deep dermis. Nerve sheath stem cells in each NCU may supply the melanocytes needed by the corresponding portion of epidermis. Transformed nerve sheath stem cells may lead to formation of occult prenatal nevi, whose histology and histogenesis may best be understood in terms of NCUs. In particular, the size and shape of a nevus may be largely determined by its NCU of origin. Six fetal nevi were detected, and 3 occult lumbosacral Mongolian spots; all in deep dermis, no later than the middle of the second trimester, mainly with a pattern of singly dispersed deep dermal melanocytes. These findings suggest that congenital (prenatal) nevi begin as intradermal nevi. In addition to explaining congenital nevi, these findings have implications for the histogenesis of acquired (postnatal) nevi and dysplastic nevi.

Characteristic alterations of cutaneous neurogenic factors in photoaged skin

BACKGROUND

Although it has been recognized that photoageing and chronological ageing differ in various morphological and biological aspects, the characteristic alterations of cutaneous neurogenic factors in photoaged skin are poorly characterized.

OBJECTIVES

To characterize cutaneous neurogenic factors, including innervation, neuropeptides, nerve growth factor and interactions of mast cells, in photoaged skin.

SUBJECTS AND METHODS

Paired biopsy specimens were obtained from sun-exposed volar forearm skin and from sun-protected dorsal upper arm skin of 20 elderly subjects. Various cutaneous neurogenic factors, including innervation, neuropeptides, neurokinin receptor, nerve growth factor, neurogenic inflammation and morphology of mast cells, were compared in sun-exposed vs. sun-protected skin quantitatively and qualitatively.

RESULTS

Cutaneous neurogenic factors associated with photoageing were characterized by a significant increase in the densities of dermal and intraepidermal nerve fibres, a correlation between epidermal innervation and the severity of photodamage, increases in the number of neuropeptidergic sensory nerve fibres in the dermis and in tissue levels of sensory neuropeptides, increases in the content of nerve growth factor, reduced expression of neurokinin receptor 1 by epidermal keratinocytes and by vascular endothelial cells and a tachykinin-specific reduction of cutaneous neurogenic inflammation. Mast cells in photodamaged skin showed several characteristic morphological features, including various degrees of activation and an intimate association with fibroblasts, which were distinct from those in sun-protected skin. Furthermore, mast cells in photodamaged skin possessed larger amounts of substance P within their granules than did those in sun-protected skin.

CONCLUSIONS

These findings document for the first time characteristic alterations of cutaneous neurogenic factors in photodamaged skin and suggest that the cutaneous nervous system may be involved in photoageing processes.

Photodamage to the cutaneous sensory nerves: role in photoaging and carcinogenesis of the skin?

Chronic exposure to ultraviolet radiation (UVR) plays a significant role in aging and carcinogenesis of the skin. Sensory nerve fibers densely innervate all layers of the skin and get in close anatomical as well as functional contact with cellular components of the epidermis and dermis. In this review, we address the impact of acute and chronic UVR exposure on the cutaneous sensory nervous system and its mediators. We suggest that skin cell-derived nerve growth factor (NGF) and skin nerve-derived neuropeptides such as substance P (SP) and calcitonin gene-related peptide (CGRP) may play a central role in intrinsic aging as well as extrinsic (photo-) aging of the skin. In addition, we discuss the possible role of these mediators in photocarcinogenesis.

The benefits and risks of long-term PUVA photochemotherapy

In 1974 a new photobiologic principle i.e. light + drug, called photochemotherapy was discovered in Boston and immediately confirmed in Vienna. Psoralen + UVA (PUVA) photochemotherapy has now been applied to the treatment of more than 24 heterogeneous groups of diseases, especially psoriasis and mycosis fungoides. After 24 years of experience in thousands of patients with psoriasis and 23 other skin disorders, virtually the only risk is the development of squamous-cell carcinomas. This risk is low with two exceptions: previous history of treatment with ionizing radiation or inorganic trivalent arsenic, and patients with recalcitrant psoriasis who require continuous treatment for many years. In a recent report from a large USA clinical trial, melanoma developed in a few patients with psoriasis treated with PUVA. This prospective clinical trial did not have a control population, and therefore, the conclusion that PUVA can cause melanoma is tentative.

Epidermal innervation correlates with severity of photodamage. A quantitative ultrastructural study

Intraepidermal nerve fibers were studied by electron microscopy in chronically photodamaged preauricular skin and in paired sun-protected postauricular sites of 20 Caucasian women aged 56-70 years. As previously reported, basal keratinocytes in the sun-exposed skin showed various degrees of degenerative changes including intracellular vacuolar structures and widened intercellular spaces. Neurites were frequently closely apposed to basal keratinocytes in preauricular sun-exposed skin, but were observed less than 10% as often in sun-protected postauricular skin. When degree of epidermal photodamage was quantified by means of the number of degenerated keratinocytes per 100 keratinocytes in the basal layer, the number of intraepidermal nerve fibers was significantly correlated by linear regression analysis to the severity of epidermal photodamage (r = 0.913) independent of anatomical sites. These results demonstrate for the first time a correlation between degree of epidermal innervation and chronic photodamage and suggest the possibility of neural involvement in the pathophysiology and/or repair of photodamaged skin.

Photochemotherapy beyond psoriasis

Photochemotherapy involves the therapeutic use of nonionizing radiation in combination with a photosensitizing chemical to trigger a photochemical reaction that mediates a beneficial effect. The successful introduction and widespread use of psoralen photochemotherapy (PUVA) in the management of psoriasis was the chief stimulus for recent interest in the therapeutic use of nonionizing radiation in various other dermatoses. This article discusses the expanding spectrum of diseases responding to PUVA therapy. More than 30 conditions such as atopic dermatitis, mycosis fungoides, vitiligo, the photodermatoses, chronic graft-versus-host disease, and granuloma annulare have been successfully treated with oral psoralen photochemotherapy. Various mechanisms of response to treatment are discussed including photoimmunologic effects, selective cytotoxicity, alterations of cell function, and stimulation of melanocytes. Finally, the limitations to the use of PUVA therapy are identified and its future use in other cutaneous and systemic diseases are discussed.

Ultraviolet radiation abolishes cutaneous nerve stainings with two axon-specific antibodies in guinea-pig skin

Cutaneous nerve fibers in guinea-pig skin were histochemically stained with two specific antibodies against different axonal proteins, a newly available protein gene product 9.5 and neuron-specific enolase. A semi-quantitative analysis revealed that the density of nerve fibers positive for either antibody was reversibly decreased following a single exposure to medium wave length ultraviolet (UVB) radiation and psoralen plus long wave ultraviolet (UVA) radiation (PUVA). UVA radiation alone did not markedly affect nerve fiber staining. The UVB/PUVA-induced nerve changes were augmented and prolonged following multiple exposures to UVB and PUVA. Nerve fiber staining was not altered by topical application of corticosteroids. Our findings suggest that both UVB and PUVA can alter the cutaneous innervation density.

A semiquantitative light and electron microscopic analysis of histopathologic changes in photochemotherapy-induced freckles

Photochemotherapy(PUVA)-induced freckles were found in 25 patients (41%) who had received more than 1,000 J/cm2 of PUVA. The patients had been treated with PUVA for more than 2 years, with more than 150 exposures before PUVA lentigines appeared on the thighs, the upper arm, the mid-lower arm, the waist, and the buttocks. The histopathology of these freckles was analyzed by light and electron microscopy. Light microscopy showed an increased amount of pigment and melanophages and increased numbers and size of melanocytes. The keratinocytes often displayed atypical features such as enlarged nuclei, giant size, or fibrillar degeneration. Homogenization of the papillary dermis was observed in 11 patients. The activation of melanocytes was confirmed electron microscopically, and pathological features such as large amounts of lipid droplets and lysosome-melanosome complexes within the melanocytes were seen. The Langerhans cells were mostly normal, whereas the keratinocytes showed cytolytic changes, fibrillar degeneration, and vacuolization. A close follow-up of patients with prolonged PUVA treatment is recommended.

Photochemotherapy (PUVA) in the treatment of urticaria pigmentosa

Eight patients received PUVA for mastocytosis. Five women had typical adult-onset urticaria pigmentosa, without evidence of systemic disease. Another woman had suspected hepatic involvement while the remaining female had early-onset familial urticaria pigmentosa with morphologically atypical mast cells. The only male patient had cirrhosis with hepatic deposits of mast cells in addition to polycythaemia rubra vera. In all patients, except the man with systemic disease, there was reduced pruritus and wealing and partial to almost complete fading of the macules. The manifestations of urticaria pigmentosa recurred after treatment was discontinued. In both lesional and uninvolved skin there was no significant change in either the mean mast cell counts or mast cell ultrastructure after an average of twenty-seven PUVA exposures. In addition, PUVA did not cause a significant alteration in the histamine content of the skin. The beneficial effect of PUVA in urticaria pigmentosa therefore does not appear to be directly related to a change in mast cell numbers or morphology, or to the histamine concentration in the skin.

Vitiligo with an inflammatory erythema in Vogt-Koyanagi-Harada disease: demonstration of filamentous masses and amyloid deposits

A 35-year-old Japanese woman developed vitiligo with an inflammatory erythema on the face two years after the onset of bilateral uveitis and dysacusis. Histological studies of the lesions revealed the loss of pigment and dense mononuclear cell infiltration around the blood vessels. At the dermo-epidermal junction area, a few eosinophilic masses were found. Electron microscopy disclosed that the masses were filamentous masses (colloid bodies) and amyloid. After the oral administration of prednisolone, the erythema subsided, and the vitiligo showed some repigmentation.

Melanocyte changes following PUVA therapy

The effect of psoralens and long-wave ultraviolet light (PUVA) therapy on human melanocytes was studied at the ultrastructural level. The results demonstrate an increase in the number of melanocytes, a change in their location, and an increase in the size of the melanosomes. Significantly, some of the melanocytes extend in narrow columns deep into the dermis, limited by the basal lamina, while others are found free within the dermis. Other morphologic alterations in the melanocytes consist of abnormal mitochondria, a swollen and distorted endoplasmic reticulum, and large lysosomes. The aforementioned abnormalities were also present in PUVA-treated skin that was examined 15 months after the treatment had been stopped.

Ultrastructural and cell surface changes of human psoriatic skin following Goeckerman therapy

Ultrastructural and cell surface studies of skin in psoriatic patients prior to and after Goeckerman therapy (crude coal tar and UVB-light) have demonstrated significant cellular changes following this treatment: hyperactivity of melanocytes with melanosome polymorphism, increase of desmosomes, tonofilaments, keratohyaline granules, a decrease in mitochondria, keratinosomes, polysomes, dark cells and a reduction in size of nuclei and nucleoli. The enlargement of intercellular spaces and the redundancy of basement membrane were also reduced. Langerhans cells were moderately decreased and exhibited a normal ultrastructural pattern. No significant changes in cutaneous nerve distribution or morphology were observed in these cases. Scanning electron microscopy following treatment revealed a regular surface and orientation of corneocytes, with flattened surfaces; and a reduction of their ridges on the surfaces, as well as of the intercellular spaces and red blood cells. These findings indicate that Goeckerman therapy restored the ultrastructural and cell surface pattern in the psoriatic skin by inducing orthokeratinogenesis, development of the tonofibrillar-desmosome system, and decrease in mitochondria, nuclei and nucleoli.

Biological changes of human cutaneous nerves caused by ultraviolet irradiation: an ultrastructural study

Three white male volunteers were irradiated by long wave ultraviolet (UV-A) and by solar simulating radiation (SSR). An acute change of cutaneous nerves was found following a single exposure of UV-A irradiation. Non-myelinated Schwann cells and perineural cells in the papillary and reticular dermis were degenerated. Axoplasms appeared to be electron-dense, but were less affected than Schwann cells. After long-term repeated exposure, many free nerve endings were found in the dermo-epidermal junction above the basal lamina and some of them made a terminal enlargement. In one instance an axon made a swelling. Dermal free nerve endings also seemed to be increased in number and some of them were situated immediately beneath the melanocytes which were active in melanogenesis. Multiplication of basal lamina of the Schwann cells and perineural cells was observed. Amorphous material was precipitated around the non-myelinated Schwann cells. After a single exposure of SSR irradiation the degeneration of Schwann cells and axons in the dermo-epidermal junction was less severe than after UV-A and changes were minor in the reticular dermis. After repeated exposure, intra-epidermal proliferation of free nerve endings was detected. Dermal nerves were slightly affected. No intra-epidermal free nerve endings were observed in controls. The intra-epidermal proliferation of free nerve endings was confirmed following repeated UV exposures. Melanocyte-nerve association is suggested to be the cause of stimulating melanocyte activity.